Anthony G. Fane

Heat and mass transfer in membrane distillation

Abstract Equations for heat and mass transfer in membrane distillation (MD) have been developed and tested experimentally. The concept of temperature polarisation is introduced and shown to be important in the interpretation of experimental results. Vapour transport through the membranes tested is reasonably described by combined Knudsen and molecular diffusion. The significance of temperature polarisation in the design and operation of large-scale MD modules is discussed, and hollow fibre and tubular systems shown to be potentially the most effective.

New All‐Vanadium Redox Flow Cell

A laboratory-scale cell was constructed to test the performance of V(II)/V(III) and V(IV)/V(V) half-cells in an all-vanadium redox battery. Graphite plates were used as electrodes, and the membrane was manufactured from a sulfonated polyehylene anion-selective material. The average charging efficiency of the cell was over 90 percent. Stability tests on the reduced and oxidized electrolytes, measured over the temperature range of -5 C to 60 C, showed no accelerated decomposition at high temperatures and no crystallization at the lower temperatures. After prolonged usage, however, a slow deterioration of the positive electrode and the membrane was observed. 9 references.

Fouling transients in nominally sub-critical flux operation of a membrane bioreactor

Abstract The development of fouling in a cross-flow microfilter coupled to an anaerobic bioreactor has been studied. The membrane bioreactor (MBR) was operated at a fixed flux substantially lower than the nominal ‘critical flux’ of the feed, measured by flux-stepping. A characteristic two-stage transmembrane pressure (TMP) profile was observed with an initially extended period of slow TMP rise followed by a sudden transition to a rapid TMP rise. This TMP profile was reproducible and depended on the imposed flux. The generic nature of this behaviour was confirmed in the MBR literature. Membrane autopsy revealed significant fouling by extracellular polymeric substances (EPS) and an uneven distribution of EPS and fouling resistance. The initial and gradual TMP rise is believed to be caused by the deposition of this EPS. The sudden rise in TMP is believed to be caused by local flux in some regions of the membrane increasing (to maintain the fixed average flux) and exceeding the critical flux of the dominant foulant (biomass). Strategies to extend the sustainable operating period are discussed.

The use of gas bubbling to enhance membrane processes

The introduction of gas/liquid two-phase flow has been shown to significantly enhance the performance of some membrane process applications. This paper reviews the state-of-the-art of this technique. In particular, the review focuses on the use of gas bubbles and slugs in microfiltration and ultrafiltration with flow inside tubes and fibres, across flat sheets and outside fibres. Examples discussed are applications in biotechnology, bioseparations and water and wastewater treatment. Some practical issues and future trends are addressed.

Colloidal interactions and fouling of NF and RO membranes: A review

Colloids are fine particles whose characteristic size falls within the rough size range of 1-1000 nm. In pressure-driven membrane systems, these fine particles have a strong tendency to foul the membranes, causing a significant loss in water permeability and often a deteriorated product water quality. There have been a large number of systematic studies on colloidal fouling of reverse osmosis (RO) and nanofiltration (NF) membranes in the last three decades, and the understanding of colloidal fouling has been significantly advanced. The current paper reviews the mechanisms and factors controlling colloidal fouling of both RO and NF membranes. Major colloidal foulants (including both rigid inorganic colloids and organic macromolecules) and their properties are summarized. The deposition of such colloidal particles on an RO or NF membrane forms a cake layer, which can adversely affect the membrane flux due to 1) the cake layer hydraulic resistance and/or 2) the cake-enhanced osmotic pressure. The effects of feedwater compositions, membrane properties, and hydrodynamic conditions are discussed in detail for inorganic colloids, natural organic matter, polysaccharides, and proteins. In general, these effects can be readily explained by considering the mass transfer near the membrane surface and the colloid-membrane (or colloid-colloid) interaction. The critical flux and limiting flux concepts, originally developed for colloidal fouling of porous membranes, are also applicable to RO and NF membranes. For small colloids (diameter≪100 nm), the limiting flux can result from two different mechanisms: 1) the diffusion-solubility (gel formation) controlled mechanism and 2) the surface interaction controlled mechanism. The former mechanism probably dominates for concentrated solutions, while the latter mechanism may be more important for dilute solutions. Future research needs on RO and NF colloidal fouling are also identified in the current paper.

A review of fouling and fouling control in ultrafiltration

Abstract This paper discusses the properties of ultrafiltration (UF) membranes which make them susceptible to fouling. Various types of flux decline are described from early usage to long-term effects. For protein UF it is shown that flux decline occurs due to protein deposition, and that this depends on membrane and solute type, solution environment and operating conditions. Attempts to model UF fouling are reviewed and selected examples of fouling control are also described.

Spacer characterization and pressure drop modelling in spacer-filled channels for ultrafiltration*

A correlation has been developed that allows the characterization and design of net-like feed channel spacers in any combination of geometric characteristics: angle, mesh size, thickness, strand size and voidage. Flow visualization was used to determine the flow path in a spacer-filled channel and to assess the effect of spacer characteristics on fluid mixing. Pressure losses in the spacer-filled channel have been modelled by inclusion of viscous drag on the channel walls and spacer, form drag of the spacer and kinetic losses due to directional flow change. This semi-empirical model permits evaluation of spacer performance. Grober’s mass transfer laminar flow correlation for developing concentration and velocity profiles was modified to account for spacer geometry. The equation obtained predicts mass transfer in spacerfilled channels with a deviation of less than 10% for the majority of spacers tested. The penalty for improving flux is increased pressure loss along the channel. An illustrative economic analysis which includes operating costs (which are pressure drop related) and capital costs (which are flux related) is used to identify optimal spacer designs for ultrafiltration.

Particle deposition during membrane filtration of colloids: transition between concentration polarization and cake formation

Abstract The transition from concentration polarization to cake formation has been studied for the membrane filtration of colloidal silica by imposing flux and observing the system response. A critical flux ( J crit ) has been measured, below which transmembrane pressure drop, Δ P , is stable for increasing and decreasing flux. The flux–pressure profiles for operations below J crit show little (for MF) or negligible (for UF) hysteresis. Above J crit the pressure has a period of instability for increasing and decreasing flux, and there is significant hysteresis. It appears that once J crit is exceeded, the colloids in the polarized layer form a consolidated cake structure that is slow to depolarize and which reduces the flux. Evidence for cake deposition was obtained from electron micrographs. The depolarization can be increased by crossflow, by washing, and increasing pH. It was observed that the slow incrementation of flux to a given high value can result in significantly lower Δ P than the direct application of that flux. These differences are ascribed to formation of a stagnant, highly concentrated layer near the membrane surface due to consolidation and aggregation of solute resulting from very rapid flux increases.

The effect of shell side hydrodynamics on the performance of axial flow hollow fibre modules

Fluid flow and mass transfer experiments have been performed on axial flow hollow fibre modules of varying packing density (32 to 76%). Shell-side pressure drop was found to be proportional to (flowrate)n, where n varied from about 1.1 at high packing density to 1.5 at low packing density, for shellside Reynolds numbers < 350. Assuming an Ergun-type pressure drop relationship it was found that for packing densities < about 50% the inertial (turbulent) losses exceeded the viscous (laminar) losses. Inspection of cross-sections taken from the middle of modules revealed non-uniform fibre packing with regions of high and low packing density. The cross-sections also change along the length of the module. It is inferred that, in addition to axial flow along fibres, there is also a degree of stream splitting which provides transverse flow across fibres as fluid continuously seeks preferential paths through regions of lower packing density. The presence of transverse flow would explain the higher than expected velocity exponent. Mass transfer experiments involving the removal of oxygen from water flowing through the shell to a sweep gas in the fibre lumens produced higher than expected shell-side mass transfer coefficients. The results are correlated within ± 15% by Sh = (0.53 − 0.58φ)Re0.53Sc0.33. The exponent on Re is consistent with entry region conditions, caused by repeated stream splitting and transverse flow. Compared with mass transfer predicted for axial flow through a uniformly packed shell the experimental results are up to 2× higher, with the most significant enhancement at the lower packing densities. The implication of this work is that module design requires a more sophisticated approach than the traditional assumption of laminar flow through parallel axial ducts.

Effect of pore size distribution and air flux on mass transport in direct contact membrane distillation

Abstract The concept of mass transfer regions within the membranes was introduced to study the mass transport in membrane distillation processes. Mass transfer model for direct contact membrane distillation (DCMD) was derived to examine the influence of pore size distribution and air fluxes on water vapor fluxes across the membranes. The pore size distributions of the membranes were determined by field emission scanning electron microscopy (FESEM) and the image analysis program. DCMD experiments with pure water were carried out under laminar and turbulent flow conditions so as to compare the experimental results with the predictions. The calculation results showed that Knudsen and transition regions were found in the membranes studied, while the transition region was the major contribution to mass transport. The model including the effect of pore size distribution and air fluxes predicted water fluxes with the average discrepancy 5% of the experimental results. The mass transfer analysis indicated that the influence of pore size distribution and air fluxes on water fluxes was insignificant. Therefore, the mass transfer model with the assumptions of air trapped in membrane pores and single pore size is adequate to describe mass transport in DCMD. The concept of mass transfer regions was also applied to analyze the effect of pore size distribution on flux in vacuum membrane distillation and gas permeation.